The present invention relates generally to compound flip-flops, superconducting bidirectional counters, and analog-to-digital converters. More particularly, the present invention relates to high-speed, high-resolution truly bidirectional analog-to-digital converters employing superconducting Josephson junctions.
High-performance analog-to-digital converters are required in a variety of electronic devices. Two of the most important measures of an A/D converter's performance are the number of samples converted per second or speed of conversion and resolution measured by the smallest increment in change that can be detected in the analog signal. A/D conversion with a resolution greater than 8 bits and an effective aperture on the order of a picosecond has not been achieved so far. In conventional state of the art semiconductor electronics, flash converters can achieve about 7 effective bits at a few picosecond apertures. In available conventional A/D converters the trend is that the greater the number of bits N (resolution), the poorer the speed of conversion, and the maximum bandwidth falls off faster than 2.sup.-N.
Superconducting technology which employs Josephson junctions as its basic switching elements is well-suited to perform high-speed, high-resolution A/D conversion because of the unique characteristics of Josephson junctions. A Josephson junction is a bistable switching device having a very thin insulating layer between two superconducting electrodes. The Josephson junction has two states: a superconducting zero-voltage state and a resistive voltage state, in which the voltage drop across the device is equal to the energy gap of the superconducting material. When current applied to the junction is increased above the critical current of the junction, the device is switched from the superconducting state to the voltage state. This switching operation can occur in a few picoseconds; therefore the Josephson junction is a high speed switching device.
However, high speed switching (few picoseconds) in underdamped Josephson junctions (those with high shunt resistance) can be achieved only from the superconducting state to the voltage state. The reset switching from the voltage state to the superconducting state cannot be achieved by merely lowering the signal current, the circuit remains in its voltage state. This property is called latching.
To achieve non-latching fast switching from the voltage state to the superconducting state the Josephson junction has to be overdamped, i.e. should include relatively low shunt resistance. In this approach the binary information is not presented by the dc voltage, as in the case of semiconductor transistor logic, as well as in the case of superconducting latching logic. The binary information is presented by very small flux quantum pulses, which can be quite naturally generated, reproduced, amplified, memorized and processed by elementary circuits comprising overdamped Josephson junctions.
One or more of the overdamped Josephson junctions combined with one or more inductors form a logic circuit called a SQUID (Superconductive Quantum Interference Device.) The single SQUID includes an overdamped Josephson junction connected across an inductor to form a superconducting loop. Therefore, a counter employing SQUIDs is a non-latching counter. The magnetic flux in the loop increases by a small quantum when the current applied to the SQUID increases by a small and precisely repeatable increment. This quantum of flux generates a small pulse of voltage across the junction. When the current applied to the junction decreases by a like increment, the magnetic flux in the superconducting loop decreases by the small quantum of flux producing a negative voltage pulse across the junction. This explains why Josephson junction logic provides high resolution in A/D conversion. In this manner the SQUID functions as a quantizer, with the pulses being detected and counted by binary counters.
Superconducting non-latching A/D converters have almost perfect linearity because a single flux quantum is only 2.07 * 10.sup.-15 weber and the current increment or decrement is the flux quantum divided by the value of the inductor in henries.
Superconducting non-latching unidirectional counters have been proposed and partially demonstrated. See J. P. Hurrell, D. C. Pridmore-Brown, and A. H. Silver, IEEE Trans. Electron Devices, ED-27, 1887 (1980); V. K. Kaplunenko, M. I. Khabirov, V. P. Koshelets, K. K. Likharev, O. A. Mukhanov, V. K. Semenov, I. L. Serpuchenko, and A. N. Vystavkin, IEEE Trans. Magn., 25, 861 (1989); C. A. Hamilton and F. L. Lloyd, IEEE Electron Device Lett., EDL-3, 335 (1982); A. H. Silver, R. R. Phillips, and R. D. Sandell, IEEE Trans. Magn., MAG-21, 204 (1985).
In prior art unidirectional counters the relative timing of multiple signals arriving at the asynchronous logic gates was extremely critical. The analog signal is inherently fully asynchronous with respect to the A/D conversion. As a result, none of the prior art unidirectional counters were capable of counting analog signals with greater than 100 GHz repetition rates.
Several bidirectional superconducting counters also have been proposed. See R. R. Phillips, A. H. Silver, and R. D. Sandell, U.S. Pat. No. 4,646,060 (February 1987); K. K. Likharev and V. K. Semenov, IEEE Trans. on Appl. Superconductivity, 1,3 (1991); G. S. Lee, U.S. Pat. No. 5,012,243 (April 1991).
The prior art bidirectional counters were basically unidirectional counters with some extra asynchronous logic added to distinguish the down-counting from the up-counting. Therefore, the bidirectional counters had the same timing problem as the unidirectional counters.
Therefore, it is desirable to provide an improved fast and high precision analog-to-digital bidirectional converter.